Scintillation cameras are well known in the art, and are used for medical diagnostics. A patient ingests or is injected with a small quantity of a radioactive isotope whose decay emission photons are detected by a scintillation medium of the camera. The scintillation medium is commonly a thallium doped sodium iodide crystal which will emit a small flash or scintillation of light in response to stimulating radiation. The intensity of the scintillation is proportional to the energy of the stimulating photon. In order to produce the medical diagnostic image, scintillations having an energy which corresponds to the energy of the decay gamma photons of the radioactive isotope are detected by measuring the intensity of each scintillation in the crystal, and then calculating and recording the exact position of the scintillations based on the intensity values from at least three light detectors coupled to a surface of the scintillation medium and surrounding the point of scintillation.
Each light detector is a device which produces an electric signal proportional to the amount of light which enters its detector surface. If the electric gain of any light detector or photodetector is incorrectly adjusted or calibrated, an image distortion will result. Therefore, means must be provided to assure that all photodetectors are equally calibrated such that the same amount of light will result in the same amount of electric signal.
It is known in the art to carry out automatic amplification (gain) control of photodetectors using a pulsable light source, as described for example in European patent application publication No. 0,066,763. It is also known to use pulsable light sources for gain calibration in which the electric signals from the photodetectors are digitally converted in applicant's copending U.S. patent application Ser. No. 07/861,636.
In the known methods of gain calibration, the light source is pulsed long enough to produce an electrical signal which is comparable to an amount of light received during a valid scintillation. Therefore, gain calibration using the pulsable light sources cannot be carried out at the same time as the camera is placed in a radiation field and used for image recording.
Gain calibration is important to assure good position calculation, since in analog methods as in digital methods of position calculation, the accuracy is based on the relative accuracy of photodetector gain calibration. Two other factors affect position calculation (i.e. image quality), namely valid event discrimination and detector edge distortion.
In valid event discrimination, one analyzes the total amount of light received from a scintillation event and decides whether the event is the result of an emission from the isotope being imaged. If events of different energies are recorded erroneously as valid events, somewhat random image distortion results. Therefore, to maintain good image quality, prior art cameras attempt to restrict acceptance of events to only those whose `energy` or total light generation is within strict tolerances. This results in some loss of valid events whose energy is detected to be outside of the tolerances, due to camera errors.
Edge distortion is the result of light detection losses when a scintillation occurs near a boundary between two or more light detectors. The edges of the detectors are usually less sensitive to light due to optical losses. In conventional cameras, some blurring and darkening of the image can be seen near detector boundaries due to edge distortion.